Correct MariaDB proxy suffix
Correct MariaDB proxy suffix

In https://review.openstack.org/577945 the suffix was changed
from 'mariadb' to 'MariaDB', so this env var should also be
changed to match it.

Change-Id: Ic6ee98e0d74eb31b067de398de2739ed7a50d438

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs